This application describes methods and apparatus for the safety and protection of capacitor units suitable for medium and high voltage applications, for instance within switching modules (400) of a voltage source converter as may be used for HVDC or FACTs. The capacitor unit (300) has a housing (201) within which is a capacitor comprising a plastic film dielectric material, for instance a winding (100) of first and second plastic films (101, 102) with metal coatings (103, 104). A pressure sensor (302) is configured to detect if the pressure in the housing exceeds a first pressure level and a pressure relief valve (304) is configured to vent gas from the housing if the pressure in the housing exceeds a second pressure level which is higher than the first pressure level.